Conventional oil recovery involves drilling a well and pumping a mixture of oil and water from the well. Oil is separated from the water, and the water is usually injected into a sub-surface formation. Conventional recovery works well for low viscosity oil. However, conventional oil recovery processes do not work well for higher viscosity, or heavy oil.
Enhanced Oil Recovery (EOR) processes employ thermal methods to improve the recovery of heavy oils from sub-surface reservoirs. The injection of steam into heavy oil bearing formations is a widely practiced EOR method. Typically, several tons of steam are required for each ton of oil recovered. Steam heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. Steam condenses and mixes with the oil, to form an oil-water mixture. The mixture of oil and water is pumped to the surface. Oil is separated from the water by conventional processes employed in conventional oil recovery operations to form produced water.
For economic and environmental reasons it is desirable to recycle the produced water. This is accomplished by treating the produced water, producing a feedwater, and directing the treated feedwater to a steam generator or boiler and producing steam. The complete water cycle includes the steps of:                injecting the steam into an oil bearing formation,        condensing the steam to heat the oil whereupon the condensed steam mixes with the oil to form an oil-water mixture,        collecting the oil-water mixture in a well,        pumping the oil-water mixture to the surface,        separating the oil from the oil-water mixture to form produced water,        treating the produced water to form feedwater for steam generation equipment, and        converting the feedwater into steam having a quality of approximately 70% to 100% for injecting into the oil bearing formation.        
Steam generation equipment can take various forms that generally include either once through steam generators (OTSG) or boilers of various types. However, treating the produced water to form a relatively pure feedwater for steam generation is challenging. In particular, treating the produced water to retard or prevent silica scaling in purification equipment, such as evaporators, and in steam generation equipment is difficult.
Various approaches have addressed silica scaling. It is known that chemically treating water to precipitate silica will reduce the silica concentration to a level that is suitable for use in producing steam using Once Through Steam Generators (OTSG). This process is generally referred to as Warm Lime Softening followed by Ion Exchange. Silica precipitates as very fine crystals that are usually only several microns in size. These fine silica crystals are difficult to economically remove by conventional mechanical separation devices such as deep bed filters, centrifuges, hydrocyclones, and gravity settlers. Another method is to trap the silica precipitates in a magnesium hydroxide and/or calcium carbonate sludge that is created by addition of lime, magnesium oxide, and soda ash. This process has the disadvantage, however, of requiring large quantities of chemicals and producing large quantities of waste sludge. When used in this method, gravity settlers are sensitive to variations in feed chemistry and are easily upset, creating problems for downstream equipment.
It is also known to chemically treat the produced water and subject chemically-treated produced water to an evaporation process that produces a distillate which becomes feedwater to an OTSG or boiler. In particular, it is known to use an evaporator and mechanical vapor compressor to produce the distillate. In this particular approach, the pH of the produced water fed to the evaporator is raised to maintain the solubility of silica. This prevents silica based scales from fouling the evaporator heat transfer surfaces. However, there are drawbacks and disadvantages to this approach as well. The addition of caustic to raise the pH represents a significant operating cost. Mechanical vapor compression evaporators recover typically approximately 95% of the water from the de-oiled produced water. The remaining 5% yields a concentrate stream that is difficult to process. The pH is usually higher than 12, which makes the concentrate stream extremely hazardous. Any attempt to neutralize the stream causes the precipitation of silica solids which are very difficult to separate from the aqueous solution. The neutralization process is also known to release hazardous gases, such as hydrogen sulfide. These systems consequently tend to be expensive to operate and costly to maintain.